1. Field of the Invention
The present invention relates to switching systems utilizing gaseous discharge effects to conduct large amounts of electric current, and more particularly, to an improved hydrogen reservoir for use in thyratrons and other types of gaseous-discharge devices.
2. Description of Related Art
It is well known in the art to utilize gaseous discharge effects to conduct large amounts of electric current. For example, a thyratron is a type of electric switch that rapidly conducts electric current between a cathode and an anode by use of a controlled gaseous discharge. A grid is disposed between the cathode and anode. When a suitable positive voltage pulse is applied to the grid, a plasma forms in the region defined between the grid and the cathode. The anode voltage produces an electric field that penetrates the grid and attracts the electrons migrating from the cathode via the plasma. The electric field subsequently causes electrical breakdown between the cathode and the anode through the grid, and in so doing, electric current is conducted through the device. Thus, thyratrons have the ability to stand-off high voltage levels while in the non-conductive "off" state, and to conduct high current levels while in the conductive "on" state. Moreover, the switching between the off and on states can be made to occur very rapidly, i.e., in sub-microsecond time scales.
In thyratrons and other types of gaseous-discharge devices, hydrogen is the medium of ionization that allows the device to conduct large amounts of current in short periods of time. It is well known that hydrogen gas is depleted during electrical discharges. To replenish the depleted hydrogen, reservoirs are used to provide a supply of hydrogen for the thyratrons and maintain the hydrogen equilibrium pressure within the devices. Alternatively, gaseous-discharge devices may also utilize deuterium as an ionization medium, and it should therefore be appreciated that this description applies to systems utilizing either hydrogen or deuterium reservoirs.
The stabilization of the hydrogen equilibrium pressure within certain defined limits is critical to the overall performance of the gaseous-discharge device. For example, if the equilibrium pressure is too low, the anode heat dissipation may be too great causing the device to experience a "red" anode and burn a hole through the envelope of the device. Further, low equilibrium pressure could also result in "trigger failure," a condition in which the thyratron fails to go into the conductive state after the triggering positive voltage pulse is applied to the grid. Conversely, if the equilibrium pressure is too high, the voltage stand-off limit may be exceeded causing undesired voltage breakdown between the cathode and the anode. In such a situation, the device may experience uncontrolled current runaway or a "latch-up" condition.
The repetitive pulse rate at which a gaseous-discharge device may be switched is the reciprocal of the high voltage recovery time of the device. A recovery condition results from the diffusion of hydrogen ions formed during current conduction into the side walls of the device, followed by recombination of the ionized hydrogen with electrons. The recovery time is therefore the time required to "open" the switch, or the time duration of de-ionization process until a subsequent application of high voltage between the cathode and anode is possible without re-conduction through the grid. The lower the hydrogen equilibrium pressure, the faster de-ionization can occur and the faster the device recovery time.
A typical hydrogen reservoir for a gaseous-discharge device comprises a heater circuit coupled to a reservoir material, in which the reservoir material controls the hydrogen pressure in accordance with changes in temperature by the heater circuit. The reservoir material forms interstitial hydrides which adsorb/absorb hydrogen as well as release hydrogen as the temperature of the reservoir material is varied. Temperature and pressure are therefore variables that are controlled by the voltage applied to the heater circuit. During manufacture, the gaseous-discharge device is back-filled with hydrogen following high temperature processing of the device to exhaust undesirable impurities. At a set input voltage of the reservoir heater, the pressure of the device is increased by filling the device with hydrogen until a desired equilibrium pressure is reached. Thereafter, during operation of the device, the hydrogen pressure is controlled by varying the heater input voltage.
The recovery time of the gaseous-discharge device depends on the relationship between the de-ionization rate and the reservoir system parameters. By changing the input voltage applied to the reservoir heater circuit, and therefore the temperature of the reservoir, the hydrogen pressure is adjusted to achieve a desired recovery rate. An important parameter of such reservoir systems is the operating range between the minimum and maximum reservoir heater voltage. These limits are determined by various factors, including the external electrical system parameters, the device geometry, and the rate of pressure change of the reservoir with the input voltage. Assuming that the electrical system parameters and grid geometry are constants, then the acceptable operating range of the device is purely a function of pressure and temperature.
If the reservoir system has a high pressure (P) variation with respect to the reservoir heater temperature (T) variation (dP/dT), the operating range will be rather small. Furthermore, with a high dP/dT, the pressure changes drastically with correspondingly small changes in the reservoir input voltage (V). This is particularly undesirable since the device can experience unstable operation from temperature fluctuations that occur due to uncontrolled line voltage shifts. Conversely, if the reservoir system has a low dP/dT, the operating range will be relatively large. The ratio of change in pressure to change in voltage (dP/dV) corresponds generally to dP/dT, and for an ideal reservoir heater in which temperature changes linearly with input voltage, the terms are considered to be equivalent.
Ideally, the reservoir will not vary significantly in temperature during device operation when set at any given heater voltage, but in practice this is difficult to achieve. As known in the art, the reservoir material typically comprises barium, zirconium, tantalum, or titanium; however, titanium is the most frequently utilized. The titanium reservoir material may be provided in one of two forms: (1) sintered, powdered titanium hydride in a container; or (2) solid titanium. As illustrated graphically in FIG. 7, solid titanium reservoirs have relatively high dP/dV, though they are simpler and thus less costly. Sintered titanium hydride reservoirs have generally lower dP/dV than solid titanium reservoirs, and therefore provide greater operating range and stability, but are costly to manufacture.
Another significant disadvantage of prior art hydrogen reservoirs is that there is often an initial pressure swing opposite to the direction which normally results from a change in reservoir input voltage. Particularly, when the voltage on the reservoir heater is increased, there may be an initial drop in hydrogen pressure before the reservoir stabilizes to the new temperature and begins to increase the hydrogen pressure. Similarly, there may be an initial increase in hydrogen pressure when the reservoir voltage is decreased. The initial pressure swing can have catastrophic effects to the operation of a gaseous-discharge device. For example, if a gaseous-discharge device is operating at an upper limit of anode temperature and the reservoir voltage is increased to raise the equilibrium pressure, the initial drop in hydrogen pressure could result in a red anode condition. To date, the cause of the opposite pressure swing phenomenon is not completely understood, and it is therefore highly desirable to provide a reservoir structure that does not exhibit such characteristics.
One approach to regulating the dP/dV relationship is to use an additional device known as a barretter. The barretter is an electrical component coupled in series with the reservoir system, and controls the dP/dV by regulating the input voltage provided to the reservoir heater. Despite the advantages of improved stability, adding a barretter to a reservoir system increases the complexity and cost of the system, and in certain small device applications the additional element could be simply prohibitive. Moreover, the barretter requires a longer warm-up time than the associated reservoir system, rendering the device impractical for applications in which a fast start is required.
Thus, a reservoir system for a gaseous-discharge device having uniformly low dP/dV characteristics across an operating range of the device without increasing the cost and complexity of the reservoir system would be highly desirable.